Of the Trinity. Lord Vishnu is the preserver, the Lord of protection and maintenance. Vishnu ji symbolises the all-pervading protective aspect of the Supreme God. Maa Lakshmi his consort, is the Goddess of material and spiritual wealth.
Vishnu ji is often depicted as dark blue or black in colour with four arms: one hand holds a lotus; the second holds a conch; the third holds a gada; and the fourth carries a chakra (which always returns on its own after being thrown). The petals of lotus are believed to symbolise the unfolding of creation; the conch is said to symbolise that from which all existence originates; and the discus and the mace reputedly were obtained by Vishnu ji as rewards for defeating God Indra. Vishnu ji is also said to possess a special sword called Nadaka and a special bow called Sarnga and his vehicle is the swift-flying bird Garuda. His abode is Vaikuntha and he rests on the ocean on his bed of Adisesha (serpent with 1,000 heads).
There is yet another explanation of the four arms of Lord Vishnu, each depicting a warning for the mankind. When we commit a mistake the first time, the Lord presents us with a flower, i.e. the lotus, as a warning. Something which you were wanting comes your way — beauty, money, relationship, job etc and you are complimented on your achievement. That compliment is a subtle warning, that what you have is only for a while. It is in passing and will soon leave you, and the pain of it leaving will be directly proportional to the pleasure it brought your way. Hardly anyone heeds the lotus. If we tend to ignore that warning and continue committing the same mistake, as a second warning Vishnu ji blows the conch, which has a physical manifestation in the form of some physical discomfort. But normally we, as adamant beings, prefer to ignore that warning also and are given the third and a more serious warning, which is the strike of the gada. This, in the physical world, comes as a very serious blow. If we still, choose to ignore it then we are no longer believed to be worthy of continuing in that physical existence and are finally made to leave that form by the strike of the chakra.
According to Vedic mythology, Vishnu ji reincarnates on Earth from time to time, to destroy the evil in the world. There are chiefly ten incarnations, according to Vaishnavism (the sect of Hinduism worshiping Lord Vishnu), although, as per the Bhagwad Purana, there are around 24 such avatars of Lord Vishnu. Nine of these ten avatars of Lord Vishnu have already descended and the tenth, that is the Kalki avatar, is said to be descending at the end of Kalyug. These incarnations reveal the help rendered by the Divine during various stages of human evolution.
Matsya (fish): The first avatar of Lord Vishnu was the Matsya who saved sage Manu from the great deluge.
Kurma (tortoise): In the battle between the devatas and asurs when the ocean was being churned for the immortal elixir. Lord Vishnu in the form of a Kurma kept the mountain afloat.
Varaha (boar): The third avatar of Vishnu was sent to defeat Hiranyaksh, a demon who had taken the Earth(prithvi) and carried it to the bottom of the ocean.
Narasimha (man-lion): Lord Vishnu incarnates as a semi-man, semi-lion to kill Hiranyakasyapa, this saving the life of his devotee Prahlada.
Vamana (short bhrahman): Lord Vishnu took birth as vamana in the house of a bhrahman (priest) and was responsible for sending Bali to pataala, thus restoring balance.
Parasurama (the warrior with an axe): This incarnation was for the purpose of defeating the tyranny of the power-drunk kshatriyas.
Ram: Ram, one of the most popular avatars of Lord Vishnu who is the soul of the epic Ramayana.
Sri Krishna: Krishna’s contributions throughout his life include the teachings of the Bhagvad Gita to Arjun.
Kalki (a man in a white horse): At the end of Kalyug (the present era) this avatar will appear seated on a white horse with a drawn sword blazing like a comet. He will punish all evildoers in this world, destroy this world, and recreate a golden age again.
Whenever there has been an imbalance in nature and evil has outgrown its prescribed proportion, the Divine has intervened in one form or the other to bring about a balance once again. The above mentioned avatars of Lord Vishnu descended for the purpose of restoring balance in the creation, which is extremely important for its sustenance.